He was the first actor to receive a "Special Guest Star" credit; this was because his character was added to the series after the other characters' billing had been contractually set. Technically a regular and not a guest star, the credit means that he holds the world's record for making guest appearances in a single series - a record that is not likely to be broken in the near future.
As a mere 8-year-old, future producer John Lasseter, who would later work with Harris in both It's a Bug's Life and Toy Story 2, was said to be a fan of Harris, prior to watching Lost in Space, which was a childhood dream come true for John.
Jonathan Harris: (On trying his hand on being a leading man of the 1940s) I thought I was Cary Grant. Oh, I looked into the mirror, and said, 'Yes, Yes. It's Cary Grant.' And then, I pulled myself together and said, 'Are you kidding?' You're a character man.
Jonathan Harris: (In 1997) Did you know that I hang in the Smithsonian Institution? Yes, I do! In the science-fiction hall there are three pictures of me with the robot!
Jonathan Harris: (From where he was born that developed his accent) I was born in New York City, and you know something interesting? My original speech was 'Listen to me! I was born on Teddy Teddy Street and Teddy Avenue, and that's the way it is, see! Want a little coffee? Let's go and have coffee, right?' I could not play a part like that, I'd fall apart laughing on the floor. It's been so long, I'd gotten away with that.